Figure 2. — Important features of Tortugas grounds. State controlled area, and boundary of fishery stations occupied. 



of the numbers of shrimp caught in 

 standard try -net tows. These estimates 

 were begun with cruise 7. 



OBSERVATIONS ON FLEET ACTIVITY 



The fleet concentration has been ex- 

 pressed in terms of the number of boats 

 fishing in a given area. This information 

 was obtained by counting boats seen from 

 the charter vessel on each trip and from 

 airplane flights over the grounds. Counts 

 of vessels from the charter boat were 

 made at the time each drag commenced. 

 Boats within sight were counted from each 

 side of the vessel. This routine was car- 

 ried out in cruises 1 -7. After cruise 7 a 

 different method was employed, the boats 

 being counted in various sectors at any 

 time during the tow. The size of the angle 

 comprising a sector varied depending on 

 the number and grouping of boats visible. 

 The number of boats seen was plotted on 

 a chart. As much care as possible was 

 taken to avoid counting a given boat more 



than once. Maximum range of visibility of 

 boats is estimated to have been 8 miles. 

 Fishing is done at night in this fishery 

 and the boats were located by their lights. 

 Out to a radius of about 3 miles the 

 pattern of lights is clear enough to dis- 

 tinguish shrimp boats from other boats; 

 beyond that distance the lights blend and 

 possibly vessels other than shrimp boats 

 were occasionally counted. Other than 

 shrimpers there is little night traffic, 

 and few false identifications were believed 

 to have been made. On any given night 

 only part of the shrimp grounds were 

 covered by the research vessel, so that 

 the total fleet was never observed at one 

 time. 



The aerial observations were made on 

 flights out of Key West and Marathon. 

 Flight patterns were arranged to allow 

 observation of the entire grounds within 

 2 hours. A single-engine airplane was 

 used without floats, and the pilots were 

 understandably reluctant to fly far fronn 

 landing fields at night. Hence flights could 



